This application is related to drive systems for utility vehicles having a prime mover powering the drive system and an auxiliary function, e.g. a pair of hydrostatic transmissions propelling a zero-turn vehicle with a mowing deck. It is desirable that the power directed to the auxiliary function remain constant to produce uniform results, such as a constant blade speed on a mowing deck to provide an even cut. It is further desirable to maintain an overall, constant power output by the prime mover to manage energy consumption. Working against these desired outcomes, increasing demands on the drive system, e.g. terrain of increasing slope, may require the utility vehicle to apportion greater amounts of power from the prime mover to the drive system and away from the auxiliary function, lessening the quality of the auxiliary function. Alternatively, the prime mover may be called upon to provide greater amounts of power to the drive system and the auxiliary function, increasing the utility vehicle's overall energy consumption. A means for adjusting the torque output of the drive system to compensate for increasing demands, while roughly maintaining a constant power draw from the prime mover is desirable.
Torque compensation has been accomplished in the past by manual methods requiring an operator to shift a transmission or motor to a greater mechanical or hydraulic reduction to accomplish an increased torque output as load increases. Such a transition can be abrupt, leading to undesirable ride quality. Automatic torque compensation has been accomplished via hydraulically actuated shift mechanisms piloted by valves reacting to increasing system pressure, and by electrically actuated shift mechanisms in communication with pressure transducers. All of these control methods add complexity and cost not suitable to certain utility vehicles, such as riding mowers.